It is known in the art that liquid mixtures can be separated in that the liquid mixture is placed into contact with one side of a suitable polymeric membrane while a vacuum is applied to the other side of the polymeric membrane or an inert gas stream is guided past the same. Some of the components of the liquid mixture that permeate more rapidly through the polymeric membrane are continuously removed in vaporized condition from the side of the polymeric membrane in communication with the gaseous phase either by evacuation or by the inert gas stream. Non-permeating components are retained in the liquid phase accumulating therein until the desired degree of separation of components of a rapid and of a poor permeation and the desired degree of purity of the retained components of an inferior permeation, respectively, has been attained.
What is particularly noteworthy of this process, called liquid permeation or pervaporation and used for the separation of gas mixtures by means of membranes, is the fact that it also permits decomposition of such liquid mixtures into their components that cannot be separated by simple distillation because they either form azeotropes or the boiling points of the comoponents are so close as to prevent an effective and economical separation. For, in pervaporation, it is no longer the partial vapor pressure of the components above the liquid that determines the composition of the mixture in the gaseous phase, but rather the different permeability of the membrane and, hence, the selectivity thereof for the various components in the liquid mixture. On laboratory scale, for example, mixtures of benzene/cyclohexane and isopropanol/water could be separated beyond the respective azeotropic mixtures by means of pervaporation. Similarly, it was possible to separate the xylene isomers o-xylene (boiling point 144.4.degree. C.), m-xylene (boiling point 139.1.degree. C.) and p-xylene (boiling point 138.3.degree. C.) by pervaporation in laboratory.
A special position is occupied by the mixtures of the simple oxygen-containing organic compounds, e.g. of the simple alcohols, ketones, ethers, aldehydes and acids, with water. On the one hand, these compounds, frequently, are technically important substances required in large scale for the most various applications in dry and anhydrous condition; conversely, these compounds, in general, completely or largely are miscible with water forming azeotropic mixtures with water so that the separation and recovery of the anhydrous organic substances involve substantial expenditure. Many attempts have therefore been made to use pervaporation processes for the separation of such mixtures; however, the efforts so far taken have never exceeded the stage of laboratory tests. Admittedly, the prior art membranes of cellulose diacetate and triacetate that are also employed otherwise, e.g., in reversing osmosis, for some purposes have an adequate mechanical stability and a satisfactory flow of permeate; however, the selectivity thereof does not yet permit a large-scale industrial use for pervaporation. The present invention is now concerned with a multi-layer membrane having a non-porous separating layer from a first polymer and a backing layer from a second polymer, which is characterized in that the separating layer is comprised of polyvinyl alcohol or cellulose acetate.